Study On The Mechanism And Process Of Combustion Synthesis Of Si₃N₄-Based Ceramics Under High Nitrogen Pressure

The compacts are made from TiSi2 and SiC powders by dry pressing. Thermodynamics, process rule and densification mechanisms of Si3N4-based ceramics during combustion synthesis and the properties of final products are studied systematically. In addition, Mo is added in raw materials for improving the properties of products. In order to explore a low cost technique which can fabricate complex shape parts efficiently, Low-Pressure Injection Molding (LPIM) combined with Self-propagation High temperature Synthesis synchronous densification by Hot Isostatic Pressing (SHS-HIP) technique are employed to fabricate slender tubes with blind hole. Detailed works are as follows:Adiabatic temperature (Tad), decomposition pressure, Gibbs free energy, and the effect of processing parameters on conversion percentage are calculated. The results show that self propagation will take place when SiC content is lower than 57wt%. TiN and Si3N4 won't decomposition when nitrogen pressure is higher than 50MPa. Complete reaction can be realized only if nitrogen penetrates into green part from outside. The proper increase of diluent content, nitrogen pressure and porosity is helpful for high conversion percentage.According to DSC and quenching experiment, reaction mechanism is analyzed and kinetic model of reaction is established. The combustion process of TiSi2-SiC-N2 system can be divided into three phase: TiSi2 melt and spread on the surface of SiC particles; TiN and liquid Si generate from the reaction between TiSi2 and N2; Si reacts with N2 to form Si3N4 crystal nucleus and whiskers. Growth mechanisms of Si3N4 whiskers include vapor-liquid-solid, vapor-solid and evaporation-agglomeration mechanisms.Compacts with different SiC content and porosity are synthesized under certain nitrogen pressure. Conversion percentage and relative density of product are calculated. Composition and microstructure of product are analyzed by XRD and SEM. Conversion percentage increases with the increase of SiC content and nitrogen pressure. In addition, when the SiC content is higher than 35wt%, SiC is nitridized under 150MPa nitrogen pressure. The relative density of product has the highest value as the increases of SiC content and porosity. Si3N4-TiN-SiC ceramics are fabricated using optimized processing parameters which have room bend strength of 430MPa, high temperature bend strength of 150MPa, fracture toughness of 3.6 MPa·m1/2, respectively.Si3N4-TiN-MoSi2-SiC ceramics are fabricated by adding different content Mo powders into TiSi2-SiC system. The high temperature bend strength, fracture toughness and relative density of Si3N4-TiN-MoSi2-SiC ceramics are improved dramatically. The specimen added 20wt% Mo has high temperature bend strength of 170MPa, fracture toughness of 6.7 MPa·m1/2, respectively.Constant temperature oxidation tests are carried out for Si3N4-TiN-SiC and Si3N4-TiN-MoSi2-SiC ceramics at 800, 1000 and 1200℃respectively. Oxidation mechanism and kinetic are analyzed, and thermal shock resistance are investigated. The relationship between oxidation weight gain of Si3N4-TiN-SiC ceramics and time is coincidence with parabolic relation and oxidation activation energy is 122.6kJ/mol. TiN is oxidized and TiO2 generates firstly during oxidation process; then Si3N4 and SiC are oxidized. Oxidation product of N2 is removed continuously, and the surface of specimen is covered by TiO2 grains. Thermal shock experiment result shows that two of Si3N4-based ceramics have better thermal shock fracture resistance.Wax-based binder system is used in LPIM process. Compatibility and weight loss characteristics of binder system are analyzed. The influence of wicking powder is investigated and heating program of debinding is established. The experiment results show that binder components are compatible each other and fit for being removed step by step. Weight loss by siphoning agrees well with exponential relationship. Debinidng process is carried out under the condition of 12μm wicking powder and 0.04MPa nitrogen pressure and the whole time of 65 hours. There is no obvious defect in tube with wall thickness of 6mm after debinding.